The present invention relates to a method and an apparatus in a fluidized bed reactor.
The furnace of a conventional fluidized bed boiler comprises an inner section having a rectangular horizontal cross section and defined by four side walls, a bottom and a roof, in which inner section the bed material containing at least solid particulate fuel material is fluidized by means of fluidization gas introduced through the bottom, mostly by primary air required by exothermal chemical reactions in the boiler. The side walls of the furnace are typically also provided with conduits for the introduction of at least fuel and secondary air.
The walls of the furnace are usually made of panels formed of finned tubes, whereby the energy released from the chemical reactions of the fuel is used for vaporization of the water flowing in the tubes. Also, superheating surfaces are often arranged in the boiler to further increase the energy content of the steam.
When the aim is to manufacture a high-capacity boiler, a large reaction volume and a lot of vaporizing and superheating surfaces are required. The basal area of the boiler is directly proportional to the capacity of the boiler on the basis of the required volume and velocity of the fluidization area. As it is at least structurally disadvantageous to have a very long and narrow surface bottom, also the height of the boiler and the width of its bottom have to be increased in order to have a sufficiently large vaporizing surfaces on the side walls. To increase the height significantly can result in structural problems and increasing the width makes it more difficult to arrange a uniform supply of fuel and secondary air. In order to solve these problems, additional structures can be arranged inside the furnace to increase the vaporizing surfaces of the boiler.
The most conventional way to increase the vaporizing surfaces of the boiler is to arrange such on the partition walls extending from one side wall of the boiler to another. An arrangement of this kind is disclosed, e.g., in U.S. Pat. No. 3,736,908. Special openings have to be arranged in such partition walls in order to ensure the uniformity of the materials and processes in various parts of the boiler. Even if there were plenty of these openings, it is difficult, however, in the boilers having partition walls to reach the homogeneity required by the optimal efficiency and the minimization of environmental emissions. These problems are most apparent in the lower corners of the boiler being the most critical points in respect of the uniform performance of the boiler, and the number of these corners is unavoidably increased by the existence of partition walls extending from one side wall to another.
The water flow passing in two phases in the vaporizing pipes is a phenomenon difficult to control in complicated geometrical patterns. From this point of view, one problem associated with simple partition walls is that there, in contrast to the boiler""s side walls, heat energy is transferred to the wall pipes from both sides. To get the vaporization and water circulation in the partition walls in balance with the vaporization in the side walls, the size of the partition wall pipes has to be larger or they have to be located more densely than in the side walls. To arrange partition walls, which can be relatively thin, considering their height, extending from the bottom of the furnace to the top thereof, can be difficult in a high boiler in view of achieving a sufficient rigidity for the walls.
It is known from the prior art also to provide the cooled partition walls with various kinds of elements necessary for the operation of the fluidized bed boiler. For example, U.S. Pat. No. 5,678,497 and International publication WO 98/25074 disclose arrangements where means for secondary air supply are attached to the cooled partition walls.
Instead of partition walls, it is also known to arrange other kinds of auxiliary structures inside the furnace used for producing steam and possibly for other operations as well. U.S. Pat. No. 5,070,822 discloses an arrangement, in which a cylindrical concentric particle separator, the outer casing of which is formed of a heat transfer surface, is arranged inside a cylindrical furnace. In the lower portion of the same structure, there are also elements for the introduction of fuel into the furnace. U.S. Pat. No. 4,817,563 discloses an arrangement, in which cooled upward tapering structures arranged in the lower portion of the furnace covering 40-75% of the furnace bottom are used for the supply of secondary air and fuel. U.S. Pat. No. 4,947,803 discloses a fluidized bed reactor where cooled cylindrical contact units are arranged. All these arrangements are, however, quite expensive and less applicable in a large scale fluidized bed boiler, and the auxiliary vaporizing surface provided by them is less significant.
It is an object of the present invention to provide a new and improved method and apparatus in a fluidized bed reactor.
It is thus an object to provide a new technical solution, by which fluidized bed boilers of various sizes can be provided with vaporizing surfaces and the above mentioned problems and defects of the prior art solved or minimized. An object is especially to provide a system to arrange vaporizing surfaces in large fluidized bed boilers.
It is a further object to provide a structurally simple and cost-effective apparatus eliminating or minimizing the above problems.
It is a further object to provide auxiliary vaporizing surfaces in a fluidized bed boiler so that as similar vaporization conditions as possible are created on all vaporizing surfaces.
It is still a further object to provide a fluidized bed reactor, in which a good mixing of materials and uniform process conditions in the furnace in spite of the auxiliary vaporizing surfaces and thus a good combustion efficiency and reduction of emissions are achieved.
In order to fulfill these objectives, the characteristics of the method and apparatus in a fluidized bed reactor in accordance with the present invention are set forth below in the claims.
The invention is especially applicable to a fluidized bed boiler. When applying the invention, vaporizing surfaces are arranged in the fluidized bed boiler so that mostly vertical chambers are arranged inside the furnace. In this specification of the invention and in the claims, the term xe2x80x98chamberxe2x80x99 refers to a structure surrounded by walls, inside which structure, a principally closed gas volume is formed. The walls are typically made of straight water tube panels formed of finned water tubes. The height of the chambers in a fluidized bed boiler is generally about the same as the height of the furnace, preferably at least 80% of the height of the furnace. The chambers extend preferably from the bottom of the furnace to the top thereof, whereby they can be used to reinforce the furnace.
When using an arrangement according to the present invention, a desirable amount of chambers can be arranged in the furnace of the fluidized bed boiler and, therefore, the size of the boiler is not restricted by the required vaporizing surfaces. In a small boiler, there can be preferably, e.g., one or two chambers according to the present invention. In a large boiler, there are preferably a plurality of, e.g., three, four, six, eight, even up to ten or more chambers. The chambers can be arranged one after the other, in two or several rows, or in another order considered best in each particular case. In a fluidized bed boiler, preferably about 20-70%, more preferably 40-60%, of the boiler""s vaporizing surface is arranged in the chambers, according to the present invention.
The chambers arranged according to the present invention are typically two-dimensional in cross section, whereby two opposite walls thereof are spaced at a short distance from each other. Both sides of the opposite vaporizing surfaces are not substantially heated, but only one side thereof is. Therefore, the conditions for all vaporizing surfaces, i.e., for the vaporizing surfaces of the boiler walls and those of the chamber walls, are primarily the same. Thus, the water tube structures can be dimensioned in the same way as the water tube structures of the boiler""s external walls. This is a significant advantage with respect to the dimensioning of the steam circuit and risk management, especially in the case of once-through boilers.
Inside the chambers, there is typically an inner section that can be used for several purposes. E.g., support structures required by the structural strength of the chambers can be built inside the chamber, whereby the chambers can be made considerably high, if necessary. The support structures arranged in the chambers can also be used to reinforce the structural strength of the furnace of the entire boiler.
The chambers in accordance with the present invention have typically such a shape that their cross section is approximately constant for most of the height of the furnace, preferably at least in 50% of the height of the furnace. Auxiliary structures required by the various functions of the fluidized bed reactor or boiler, especially when attached to the upper and lower portions of the chambers can, however, change the shape of the chamber at that point.
By applying the arrangement according to the present invention, the fluidized bed reactor, typically the fluidized bed boiler, can be provided with more vaporizing surfaces without any need to divide the furnace into separate points by partition walls. The entire furnace bottom can, except for the separate chambers, be continuous. Therefore, the process taking place inside the furnace, typically the combustion process, needs not to be divided into parts, but the bed material can move almost freely inside the entire volume of the furnace.
The horizontal cross section of the chambers is preferably convex, i.e., as seen from inside the chamber, the angles formed of the adjacent walls thereof are less than 180 degrees. Further, the chambers are preferably spaced away from the side walls of the furnace. Thus, the chambers do not form inner corners in the furnace, which could be problematic in respect of the mixing, but all the corners created by them are outer corners as seen from the direction of the furnace. Thus, most of the volume, even in the proximity of the chambers, is free for the particles to move and their movement is not substantially restricted. In order not to restrict the movement of the particles in the furnace, each diagonal of the chambers is preferably not more than 60%, more preferably not more than 50%, of the parallel diagonal of the furnace.
Also, other structures and functions related to the fluidized bed boiler can be attached to the vaporizing chambers in accordance with the present invention. Most preferably, means for supplying secondary air are arranged in the chambers. Also, means for the introduction of fuel or limestone can be arranged in the chambers, whereby the transfer of fuel or limestone inside the chambers is preferably carried out pneumatically or by means of a feed screw arranged in a sloping position.
The chambers can be preferably formed of planar water tube panels, even if in some cases it is advantageous to use chambers having a round cross section. The cross section of the chambers has preferably the shape of a polygon, more preferably a rectangle. The cross section of the rectangle can be square, but preferably it is elongated so that the proportion of the respective lengths of the long side and the short side is at least two. A chamber having an elongated cross section is advantageous, since it provides a lot of vaporizing surface without significantly adding to the total area of the boiler""s bottom. In order to be able to arrange various structures and devices in the chambers, the distance between the opposite walls thereof should be preferably at least 0.5 m, and most preferably at least 1 m.
Also, a particle separator can be preferably arranged in one or several chambers of the fluidized bed boiler, whereby in the upper portion of the chamber, one or several openings are arranged, through which the flue gas generated in the furnace and the bed material entrained with it can flow into the inner section of the chamber. An impact separator or a cyclone separator is arranged inside the chamber for separating the flue gas from the bed material, entrained with it. The cleaned flue gas is discharged through the upper portion of the chamber and the separated bed material is returned to the furnace.
According to one preferred embodiment of the invention, the chambers containing a particle separator are square in cross section, whereby inlet conduits from the furnace are arranged in one or several side walls close to the chamber corner. Most preferably, an inlet is arranged in each side wall of the square chamber.
The chambers containing a particle separator can also have an elongated cross section, whereby two or several vortices next to each other are generated in one chamber by means of the inlet and outlet openings. There can be internal partition walls in the chamber between the various vortices or the vortices can be in the same place.
Also, heat transfer chambers can be preferably arranged connected to the chambers, e.g., superheating surfaces of wing-wall type. In this case, the inside of the chambers is provided with connecting pipes for steam, from where the superheating pipes are led outside of the chamber wall, i.e., to the furnace, so that the pipes and tube panels continue upward in the proximity of the wall and end up in the headers arranged above the roof of the furnace.
By arranging a necessary number of separate chambers in the boiler, the distance between two adjacent introduction points for fuel and secondary air can be given a desired length everywhere. Thus, homogeneous process conditions can be arranged in a completely new way even in the furnace of a larger boiler when using an arrangement according to the present invention.
Vaporizing surfaces can be arranged to a necessary extent even in a large fluidized bed boiler when using an arrangement in accordance with the present invention without either increasing the height of the furnace or impairing the mixing of the material. By adding auxiliary structures in the chambers according to the present invention, the rigidity of the boiler, the homogeneity of materials and processes can be improved and free space on the boiler""s side wall increased.